The present invention relates to the field of measurement of materials used in the fabrication of semiconductor devices. Specifically, the present invention pertains to using transmission characteristics of thin-film layers disposed on a substrate to determine the thickness of the thin-film layer.
Semiconductor wafers typically include thin-films formed on semiconductor substrates. There is a need to be able to measure and analyze characteristics of these films. Previous systems have provided a way to analyze the thickness and density of thin-films disposed on semiconductor substrates using X-ray reflectometry (XRR). For example, U.S. Pat. No. 5,619,548 and PCT Publication W01/71325 A2 (referred to herein as the ""325 application) discuss different aspects of XRR systems and are hereby incorporated by reference.
XRR systems of the prior art make use of the fact that x-rays reflected off of a thin-film disposed on a substrate are detected as having different characteristics depending on the x-ray""s angle of reflection relative to the surface of the structure. FIG. 1 shows a view of a prior art XRR system for simultaneous measurements of the reflectivity over a range of angles. As shown in FIG. 1 a source 100 generates an x-ray beam 101 that is incident upon an x-ray reflector 102, which is typically a monochromator. X-rays are then focused upon the sample being evaluated 106 which is positioned on a supporting stage 104. X-rays incident upon the sample are reflected and then detected with a position-sensitive detector 108 (such as a photodiode array).
Reflected x-rays 110 are captured in the top half of the detector 108, while the incident beam 112 can be measured by lowering the stage and reading the bottom half of the detector. By properly normalizing the two profiles (as described in the ""325 application) one can determine the reflectivity as a function of angle. Signals are generated by the detector 108, and the information contained in these signals is then used by the processor system 114 to analyze the reflectivity characteristics of the sample 106. The processor system 114 can then generate a display 116 to convey information about the sample 106 to user.
FIG. 2 shows a typical plot of angle-resolved XRR data, in a graph form which could be generated by the processor system 114. This type of graph depicts the efficiency with which monochromatic x rays are reflected from a sample, and this type of information can characterize the reflectivity of a thin-film disposed on a substrate. Specifically, FIG. 2 shows a graph for reflectivity of x-rays incident on a 358 xc3x85 cobalt thin film, on a substrate taken at 6.4 keV. The reflectivity signal shows a fringe pattern having peaks 206, and these peaks correlate to different reflection angles. It will be readily appreciated by one skilled in the art that as the thickness of the film increases the difference in the reflection angle between the peaks will decrease. For thin-films of sufficient thickness, prior systems may not be able to accurately resolve the fringe pattern, and as a result it may be difficult or impossible to determine the thickness of the thin-film. One approach for dealing with this problem is to modify the resolution of the system, but in general there is a limit to how much the resolution of the system can be increased, and further increasing the resolution of the system results in an increase in the amount of time it takes to make a measurement. (Aspects of one approach to varying the resolution of the system are disclosed in co-pending commonly assigned patent application Ser. No. 10/053,373 entitled X-RAY REFLECTANCE MEASUREMENT SYSTEM WITH ADJUSTABLE RESOLUTION, filed Oct. 24, 2001, which is incorporated herein by reference.)
One example of a semiconductor wafer structure where prior art XRR techniques are often unable to accurately determine the characteristics of a thin-film, is where a thin-film of porous SiO2 is formed on a second film, or material, which is composed of a material which is denser than SiO2. Using previous systems and methods it was often difficult, or impossible to accurately determine the thickness of the porous SiO2 material, because in many applications the SiO2 layer is thick enough that it produces a very narrow fringe pattern which is beyond the resolution of the system. What is needed is a system and method for accurately determining the thickness of a thin film layer where the thin film layer is such that it produces an fringe pattern that is cannot be accurately resolved using standard XRR systems.
Prior XRR systems utilize fringe patterns in reflectivity data to determine the thickness of a thin-film layer. In general terms, the fringe pattern is caused by the interference of x-rays reflected at the several density interfaces present in a thin-film structure, such as for a thin-film layer on a substrate. Changes in the thickness of the thin-film layer will result in changes in fringe pattern.
In contrast with prior methods which focus on using the reflectivity information to determine a fringe pattern and then use this information to determine the thickness of the thin-film, the present method and system use reflectivity information to determine transmission characteristics of the thin-film layer. The transmission characteristics are then used to determine the thickness of the thin-film. A system and method which evaluates the transmission characteristics of the thin-film, as disclosed herein, can be used to determine the thickness of the thin-film structures which could not be determined using many prior systems which utilized fringe pattern analysis.